Out of my 9 days visit to Switzerland (April-May 2018), I kept one day for mount titlis (I am staying in Lucerne). However, I understand that this is not fully covered by STP and hence, I do not want to spend money on this (I am already doing Jungfrau).

What are the other alternatives which can be done in one day from Lucerne? (Please note that I am already doing Stanserhorn, Rigi, Stoos, lake Lucerne and local lucerne. I only have 3 days for Lucerne.)

Can I do Gotthard Panorama Express? any other recommendations? Any place little far which can be done in a day?

If you want to do the Gotthard in a day take an early fast train to Lugano and come back on the GPE. It will be a long travel day and you will be doubling up on some lake travel if you have already covered from Brünnen on your Stoos trip.